In a straight tube type Coriolis flowmeter, when a vibration is applied to a straight tube (flow tube), supported at both ends, in a direction perpendicular to the straight-tube center portion axis, there is obtained between the support portions and the central portion of the straight tube a difference in displacement of the straight tube due to a Coriolis force, that is, a phase difference signal, based on which the mass flow rate is detected. The straight tube type Coriolis flowmeter has a simple, compact and solid structure (see, for example, JP 2786829 B).
In FIG. 13, a conventional straight tube type Coriolis flowmeter 1 has an outer cylinder 2, a flow tube (inner tube) 3, a counter balance (outer tube) 4, connection blocks 5, plate springs 6, a drive device 7, detectors (detecting means), and a weight (not shown). The flow tube 3 has at its both ends divergent opening portions 9 of a trumpet-like configuration. Further, the flow tube 3 has a straight tube portion 10 between the divergent opening portions 9 at both ends.
On the outer side of the straight tube portion 10 of the flow tube 3, there is provided the counter balance 4. The straight tube portion 10 of the flow tube 3 and the counter balance 4 are coaxially joined together at both ends of the counter balance 4 by the connection blocks 5. The connection blocks 5 are provided as rigid members. A double tube structure is formed by the straight tube portion 10 and the counter balance 4. The outer cylinder 2 is formed so as to be capable of accommodating the double tube structure. Both end portions of the outer cylinder 2 are formed so as to be reduced in diameter toward the divergent opening portions 9 of the flow tube 3. Both end portions of the outer cylinder 2 are welded to the divergent opening portions 9. Both end portions of the outer cylinder 2 and the divergent opening portions 9 are fixed to each other in a liquid tight fashion. Connection flanges 11 are welded to the open ends of the divergent opening portions 9. The divergent opening portions 9 are formed so as to exert a spring action.
Each plate spring 6 has a surface orthogonal to the straight tube portion 10; one end of the surface is fixed to the connection block 5, and the other end thereof is fixed to the inner wall of the outer cylinder 2. Further, the plate springs 6 are arranged so as to be orthogonal to the direction of resonance vibration. The drive device 7 is mounted to the position at the center of the flow tube 3 and of the counter balance 4. The drive device 7 drives the straight tube portion 10 of the flow tube 3 and the counter balance 4 at coupled vibration frequencies of opposite phases. The detectors 8 are mounted at positions symmetrical with respect to the drive device 7. The weight (not shown) is mounted at a position on the opposite side of the drive device 7. More specifically, the weight (not shown) is mounted at a position in the driving direction of the drive device 7. The weight (not shown) is provided so as to allow an adjustment to equalize the natural frequency of the flow tube 3 around the connection blocks 5 with the natural frequency of the counter balance 4.
In the above-mentioned construction, the resonance system including the flow tube 3 and the counter balance 4 is supported by the plate springs 6. The divergent opening portions 9 at the ends of the straight tube portion 10 extending from the resonance system are supported at the positions of the connection flanges 11. Thus, the flow tube 3 is supported at a plurality of points. In the straight tube type Coriolis flowmeter 1, constructed as described above, a fluid to be measured (not shown) is caused to flow through the flow tube 3, and in this state, the drive device is resonance-driven to detect a phase difference signal proportional to a Coriolis force with the detectors 8, whereby the mass flow rate can be measured. In the straight tube type Coriolis flowmeter 1, a standing wave is formed in the resonance system through resonance driving of the drive device 7 mentioned support points constitute vibration nodes.
In the conventional straight tube type Coriolis flowmeter 1, in order to achieve an improvement in vibration resistance and to eliminate vibration leakage, there is provided a mass point vibrated in a direction opposite to the vibration of the flow tube 3, i.e., the counter balance 4, thereby counterbalancing the vibration. Further, in the conventional straight tube type Coriolis flowmeter 1, the drive device 7 and the detectors 8 are installed not on the outer cylinder 2 but on the counter balance 4, and the counter balance 4 with the drive device 7 and the detectors 8 installed thereon is fixed not to the outer cylinder 2 but at two positions of the flow tube 3 through the intermediation of the connection blocks 5.
(It is a structure for preventing direct superimposition of noise on detectors 8 when external disturbance acts on straight tube type Coriolis flowmeter 1. Further, it is a structure for making a lower bending vibration which is the most liable to occur (the principal mode generated by external disturbance) different from the drive mode.) Further, in the conventional straight tube type Coriolis flowmeter 1, the plate springs 6 are provided to fix the connection blocks 5 in position, with the result that the directional properties of the vibration are determined. (Due to the provision of plate springs 6, a rotation center exists in each connection block 5 during vibration.)
In the conventional straight tube type Coriolis flowmeter 1, constructed as described above, the connection blocks 5 connecting the flow tube 3 and the counter balance 4 are rigid as stated above, which leads to the following problem. When an axial force is generated in the flow tube 3, local stress is generated between the pair of connection blocks 5 and between the ends of the flow tube 3 and the connection blocks 5, and in some cases, some stress remains in the flow tube 3, or there is a fear of the flow tube 3 undergoing plastic deformation.
In the following, an axial stress acting on the flow tube 3 will be described with reference to a schematic diagram. In the following description, to clarify the problem caused by the axial stress, the function (spring action) of the divergent opening portions 9 will be ignored. FIGS. 14 (a) through 14 (d) are schematic diagrams each showing the condition of the flow tube 3 and the counter balance 4 when the temperature of the fluid flowing through the flow tube 3 is raised, and FIG. 14(e) is a perspective view showing the positional relationship between the flow tube 3, the counter balance 4, the connection block 5, and the plate spring 6.
When the fluid to be measured is passed through the flow tube 3 and the drive device 7 is resonance-driven, the flow tube 3 and the counter balance 4 vibrate in loci as shown in FIG. 14(a). In a state in which the temperature of the fluid to be measured is not raised, the temperature of the whole is uniform (i.e., there is no temperature change). In this state, no axial stress has been generated yet between the pair of connection blocks 5 and between the ends of the flow tube 3 and the connection blocks 5. (During driving, however, stress due to vibration is separately exerted.)
When the fluid to be measured is continuously passed and its temperature is raised, there is generated in the flow tube 3, as a result of a temperature change, a force by which the flow tube 3 is axially expanded. In contrast, in the counter balance 4, to which the heat due to the temperature change has not been conducted to a sufficient degree, no such expansion force as that in the flow tube 3 is generated, with the result that the distance between connection blocks 5 remains substantially the same. Thus, as shown in FIG. 14(b), in the flow tube 3, there is generated, apart from the stress due to vibration, a local axial stress leading to compression.
When, thereafter, the counter balance 4, which has adapted itself to the heat due to the temperature change, expands in the axial direction, the distance between the connection blocks 5 increases in accordance therewith, and the axial stress that has generated between the connection blocks 5 is mitigated as shown in FIG. 14(c). However, in contrast, the axial stress leading to compression between the ends of the flow tube 3 and the connection blocks 5 increases, so a large local axial stress is exerted on the flow tube 3.
When the temperature of the whole becomes uniform, and the distance between the fixed ends of the flow tube 3 is increased, axial stress ceases to exist in the flow tube 3 as a whole as shown in FIG. 14(d), with the result that a stable state is attained.
FIGS. 15 (a) through 15 (d) are schematic diagrams showing the condition of the flow tube 3 and the counter balance 4 when the temperature of the fluid to be measured flowing through the flow tube 3 is lowered. FIG. 15 (a) shows a state in which the temperature of the fluid to be measured is high, and the temperature of the whole is uniform; in this state, no axial stress acts on the flow tube 3 as a whole.
When the temperature of the fluid to be measured is lowered, there is generated in the flow tube 3, as a result of the temperature change, a force by which the flow tube 3 is axially contracted.
In contrast, no change is to be observed in the length of the counter balance 4, that is, the distance between the connection blocks 5 and the distance between the fixed ends of the flow tube 3. As shown in FIG. 15(b), there is exerted on the flow tube 3 a local axial stress leading to tension.
When, thereafter, the counter balance 4, which has adapted itself to the heat due to the temperature change, is axially contracted, the distance between the connection blocks 5 is reduced in accordance therewith. As shown in FIG. 15(c), the axial stress generated between the connection blocks 5 is mitigated. However, in contrast, the axial stress leading to tension between the ends of the flow tube 3 and the connection blocks 5 increases, so a large local axial stress is exerted on the flow tube 3.
When the temperature of the whole becomes uniform, and the distance between the fixed ends of the flow tube 3 is reduced, axial stress ceases to exist in the flow tube 3 as a whole 3 as shown in FIG. 15(d), with the result that a stable state is attained.
As can be understood from the above illustration, in the conventional straight tube type Coriolis flowmeter 1, the axial stress acting on the flow tube 3 is not easily dispersed in the tube axis direction. Thus, the conventional straight tube type Coriolis flowmeter 1 is vulnerable to temperature change.